Image forming apparatus and control method of image forming apparatus

ABSTRACT

An image forming apparatus includes first and second light sensors positioned in a laser scanning system of at least one color, such that scanned light is detected by the first light sensor and then by the second light sensor and light sensing surfaces of the first and second light sensors are not parallel, and a control unit connected to the first and second light sensors and configured to determine a time difference in the timing of light detection by the first and second light sensors and to execute a color position shift operation upon determining that the time difference is greater than a first threshold value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/941,290, filed on Mar. 30, 2018, the entire contents of each of whichare incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an image formingapparatus, and a control method of the image forming apparatus.

BACKGROUND

In the related art, in a laser exposure-type color multifunctionperipheral (MFP), or a printer, a control method of performing anoperation to shift a position of printing for each color (hereinafterreferred to as “color position shift operation”), or the like, at apoint of time in which a continuous printing output or an intermittentprinting output passes a preset time, has been used. However, in thecolor position shift operation of the related art, there is a case inwhich a color position shift is too large as a result of temperaturechanges inside a laser scanning unit, caused by heat generation by apolygon mirror motor, or the like, especially when continuous printingis performed, or when the preset time is too long. In addition, in thecolor position shift operation of the related art, there is also a casein which, when the preset time is too short, color position shiftoperations are performed too frequently, and as a result, productivityof a printing output decreases, or the lifespan of a developer or aphotoconductive drum decreases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view of an information processing device accordingto an embodiment.

FIG. 2 is a diagram of a scanning system of a printing unit.

FIG. 3 is another diagram of the scanning system of the printing unit.

FIG. 4 is a block diagram of a control system of the informationprocessing device.

FIG. 5 is a diagram which illustrates an orientation of a first sensorused for horizontal synchronization.

FIG. 6 is a diagram which illustrates an orientation of the first sensorand a second sensor which detects a change in scanning position.

FIG. 7 is a diagram which describes a method of detecting a change inscanning position.

FIG. 8 is a diagram which illustrates another example of an orientationof the first sensor and the second sensor.

FIG. 9 is a timing diagram of signals of the first sensor and the secondsensor in scanning positions right after the time color position shiftoperation was performed.

FIG. 10 is a timing diagram of signals of the first sensor and thesecond sensor in scanning positions after some passage of time from thetime color position shift operation was performed.

FIG. 11 is a flowchart which illustrates an example of power-onprocessing which checks for the need for color position shift operationaccording to an embodiment.

FIG. 12 is a conceptual diagram showing example patterns produced duringthe color position shift operation.

FIG. 13 is a flowchart which illustrates steps of the color positionshift operation.

FIG. 14 is a flowchart which illustrates steps carried out to determinewhether to perform color position shift operation or the correctionprocessing.

DETAILED DESCRIPTION

An image forming apparatus includes first and second light sensorspositioned in a laser scanning system of at least one color, such thatscanned light is detected by the first light sensor and then by thesecond light sensor and light sensing surfaces of the first and secondlight sensors are not parallel, and a control unit connected to thefirst and second light sensors and configured to determine a timedifference in the timing of light detection by the first and secondlight sensors and to execute a color position shift operation upondetermining that the time difference is greater than a first thresholdvalue.

Hereinafter, an information processing device, an information processingsystem, and a control method of the information processing deviceaccording to an embodiment will be described with reference to drawings.In the following embodiment, a multifunction peripheral is depicted asan example of the information processing device.

FIG. 1 is an external view of an information processing device 100according to the embodiment.

As illustrated in FIG. 1 , the information processing device 100 is amultifunction peripheral which can forma toner image on a sheet. Thesheet is, for example, paper, or the like. The sheet may be any sheet onwhich the information processing device 100 can form an image.

Further, the information processing device 100 can read an image on asheet. The sheet may be any sheet from which the information processingdevice 100 can read an image. The information processing device 100generates digital data by reading an image illustrated on the sheet, andgenerates an image file.

The information processing device 100 is provided with a display 110, acontrol panel 120, a printing unit 130, a sheet accommodating unit 140,an image reading unit 150, a plurality of sensor units 160Y-160K (shownin FIG. 3 ), sensors 210, a control unit 170, and a storage unit 180.The printing unit 130 of the information processing device 100 may be adevice which fixes a toner image onto a sheet. According to theembodiment, the printing unit 130 will be described as an example of adevice which fixes a toner image.

The display 110 is an image display device such as a liquid crystaldisplay, or an organic electroluminescence (EL) display. The display 110displays various information related to the information processingdevice 100. In addition, the display 110 outputs a signal correspondingto an operation input performed by a user to the information processingdevice 100. The display 110 receives an operation of the user.

The control panel 120 includes a plurality of buttons. The control panel120 receives an operation inputs by the user. The control panel 120outputs a signal according to an operation performed by the user to theinformation processing device 100. The display 110 and the control panel120 may be configured as an integrated touch panel.

The printing unit 130 executes image forming processing. In the imageforming processing, the printing unit 130 forms an image on a sheetbased on image information generated by the image reading unit 150, orimage information received from outside of the device through acommunicating path. In addition, as will be described later, theprinting unit 130 includes a light source, a driving unit of the lightsource, a polygon mirror, a driving unit of the polygon mirror, or thelike. In detail, the printing unit 130 includes, for example, fourphotoconductive drums (shown in FIG. 2 ), intermediate transfer belt ITBhaving an endless belt shape, four primary transfer rollers PTRY-PTRK(shown in FIG. 2 ). The primary transfer rollers PTRY-PTRK contact asurface of the intermediate transfer belt ITB. The four photoconductivedrums (shown in FIG. 2 ) correspond to yellow color, magenta color, cyancolor and black color respectively. The printing unit 130 forms tonerimage on each of the photoconductive drums respectively, transfers thetoner image formed on the photoconductive drums onto the intermediatetransfer belt ITB (to execute a primary transfer) in collaboration withthe primary transfer rollers PTRY-PTRK, and then further transfer thetoner image on the intermediate transfer belt ITB onto a sheet (toexecute a secondary transfer).

The sheet accommodating unit 140 accommodates sheets which are subjectedto the image forming by the printing unit 130.

The image reading unit 150 reads an image of a reading target asbrightness and darkness of light. For example, the image reading unit150 reads an image printed on a sheet set as a reading target. The imagereading unit 150 records image data read from the reading target. Therecorded image data may be transmitted to another information processingdevice through a network. The recorded image data may be formed as animage on a sheet using the printing unit 130.

Each of the plurality of sensor units 160Y-160K includes a pair of lightdetecting sensors, detects light radiated from the light source providedin the printing unit 130, and outputs the detection result to thecontrol unit 170.

The sensors 210 acquire density values of a toner image formed on thesurface of intermediate transfer belt ITB when carrying out thealignment processing. When visible images are formed on the intermediatetransfer belt ITB, the sensors 210 acquire different density values. Inone embodiment, the sensors 210 are arranged at front and rear sides ofthe intermediate transfer belt ITB, such that from the viewpoint of FIG.2 , the sensor 210 at the rear side is behind and obscured by the sensor210 at the front side. The control unit 170 controls the informationprocessing device 100 according to a control application program orsetting stored in the storage unit 180. The control unit 170 performs acolor position shift operation based on the detection result output bythe sensor units 160Y-160K. Here, the color position shift operation isan operation to correct a shift in the printing position of each colorwhich occurs due to a position shift, or the like, of scanning systemsof optical systems of two or more colors, which are provided in theprinting unit 130. The color position shift operation will be describedlater.

The control unit 170 includes a processor and a memory. The processorperforms the operation of functional units described herein by executingprograms or the like stored in the memory or the storage unit 180. Theprocessor is, for example, a central processing unit (CPU).Alternatively, the functions of the control unit 170 can be realized bya control circuit, an ASIC, a programmed processor, and a combinationthereof. The memory is, for example, volatile memory, non-volatilememory, or a combination thereof.

The storage unit 180 stores a control application program, setting,various threshold values, a scanning speed of the light source, and foreach of the sensor units 160Y-160K, a time measurement representing anamount of time that elapses between light detections by the pair oflight detecting sensors therein, and an angle θ formed by the pair oflight detecting sensors. The storage unit 180 is, for example, a flashmemory, a hard disk drive (HDD), or a solid state drive (SSD).

An example structure of the scanning system of the printing unit 130will be described below, using FIGS. 2 and 3 . FIG. 2 is a diagram whichillustrates the structure of the scanning system of the printing unit130 according to the embodiment. FIG. 3 is another diagram whichillustrates the structure of the scanning system of the printing unit130 according to the embodiment which is viewed from above.

As illustrated in FIG. 2 , the scanning system of the printing unit 130includes a first light source (not illustrated), an fθ lens 1321, asecond light source (not illustrated), an fθ lens 1322, a third lightsource (not illustrated), a polygon mirror 1301, an fθ lens 1323, afourth light source (not illustrated), an fθ lens 1324, reflectionmirrors 1341, 1342, 1351, 1352, 1361, 1362, 1363, 1364, 1381, 1382, 1383and 1384, and sensor units 160Y, 160M, 160C, and 160K. The sensor units160Y, 160M, 160C, and 160K are positioned adjacent to the end ofreflection mirrors 1381, 1382, 1383 and 1384 in the scanning direction,respectively, as shown in FIG. 3 . The configuration of the embodimentis an example of including four light sources, and it is not limited tothis. The light sources may be two or more. In addition, in the exampleillustrated in FIG. 3 , the sensor unit is provided for each color;however, the sensor unit may be provided for just one color, e.g., theblack color.

The respective first light source, second light source, third lightsource, and fourth light source are semiconductor lasers, for example.The first light source is a light source corresponding to black color,for example. The second light source is a light source corresponding tocyan color, for example. The third light source is a light sourcecorresponding to magenta color, for example. The fourth light source isa light source corresponding to yellow color, for example.

The polygon mirror 1301 polarizes light beams which are input byrotating under a control of the control unit 170.

The reflection mirrors 1341 and 1351 reflect a light beam which ispolarized by the polygon mirror 1301.

A light beam reflected by the mirror 1351 is input to the reflectionmirror 1361, and then the reflection mirrors 1361 and 1381 guide thelight beam toward a photoconductive drum for the black color. The sensorunit 160K is used for horizontal synchronization and a change inscanning position in the sub-scanning direction with respect to thelight beam reflected by the reflection mirror 1381.

The light beam reflected by the reflection mirror 1351 is also input tothe reflection mirror 1362, and then the reflection mirrors 1362 and1382 guide the light beam toward a photoconductive drum for the cyancolor. The sensor unit 160C is used for horizontal synchronization and achange in scanning position in the sub-scanning direction with respectto the light beam reflected by the reflection mirror 1382.

The reflection mirrors 1342 and 1352 reflect the light beam which ispolarized by the polygon mirror 1301.

The light beam reflected by the reflection mirrors 1342 and 1352 isinput to the reflection mirror 1363, and then the reflection mirrors1363 and 1383 guide the light beam toward a photoconductive drum for themagenta color. The sensor unit 160M is used for horizontalsynchronization and a change in scanning position in the sub-scanningdirection with respect to the light beam reflected by the reflectionmirror 1383.

The light beam reflected by the reflection mirrors 1342 and 1352 isinput to the reflection mirror 1364, and then the reflection mirrors1364 and 1384 guide the light beam toward a photoconductive drum for theyellow color. The sensor unit 160Y is used for horizontalsynchronization and a change in scanning position in the sub-scanningdirection with respect to the light beam reflected by the reflectionmirror 1384.

Subsequently, a configuration example of a control system of theinformation processing device 100 according to the embodiment will bedescribed.

FIG. 4 is a block diagram of hardware components of the control systemof the information processing device 100 according to the embodiment. Asillustrated in FIG. 4 , the information processing device 100 isprovided with the display 110, the control panel 120, the printing unit130, the sensor unit 160Y-160K, the control unit 170, and the storageunit 180.

The printing unit 130 is provided with a driving unit 131, a lightsource 132, a driving unit 133, and a polygon mirror 1301.

The sensor unit 160Y for yellow color is provided with a first sensor161Y (also referred to herein as a “first light detecting sensor”), anda second sensor 162Y (also referred to herein as a “second lightdetecting sensor”). The sensor unit 160M for magenta color is providedwith a first sensor 161M, and a second sensor 162M. The sensor unit 160Cfor cyan color is provided with a first sensor 161C, and a second sensor162C. The sensor unit 160K for black color is provided with a firstsensor 161K, and a second sensor 162K. Hereinafter, reference numeral160 is used for representing the sensor units 160Y-160K, referencenumeral 161 is used for representing the first sensors 161Y-161K, andreference numeral 162 is used for representing the second sensors162Y-162K.

The control unit 170 is configured to function as a change amountdetecting unit 171, a counter 172, and an image forming unit 173.

In the example illustrated in FIG. 4 , one light source in the pluralityof light sources is illustrated by being extracted. The one light sourceis the first light source for black color (FIG. 2 ), for example.

The driving unit 131 drives the light source 132 according to a controlof the control unit 170.

The driving unit 133 drives the polygon mirror 1301 according to acontrol of the control unit 170.

Each of the first sensors 161Y, 161M, 161C and 161K is used forhorizontal synchronization, and outputs the detection result to thechange amount detecting unit 171 of the control unit 170.

Each of the second sensors 161Y, 161M, 161C and 161K is configured todetect a change in scanning position in the sub-scanning direction withrespect to the light beam, and outputs the detection result to thechange amount detecting unit 171.

For example, in a case of black color, the change amount detecting unit171 obtains a time difference between the detection of the light beam bythe first sensor 161K and the detection of the light beam by the secondsensor 162K. For example, the change amount detecting unit 171 obtainsthe time difference by causing the counter 172 to start counting whenthe first sensor 161K outputs the detection result, and counting a timeuntil the second sensor 162K outputs the detection result. The changeamount detecting unit 171 determines whether or not the time differenceis large enough to require color position shift operation by comparingthe obtained time difference with a predetermined threshold value storedin the storage unit 180. In addition, when information denoting adetected point of time (e.g., a time stamp) is included in the detectionresult output by the sensor 160K, the change amount detecting unit 171may obtain the time difference from the detected point of time.

The counter 172 performs a start of counting, and ending of countingaccording to a control of the change amount detecting unit 171.

The image forming unit 173 performs image forming processing bycontrolling the printing unit 130. The image forming unit 173 causes theprinting unit 130 to form an image according to a received instruction.The received instruction is, for example, position shift processing withrespect to each color. The received instruction may be printing,copying, faxing, or the like, received from a user or a maintenanceperson.

Subsequently, an example of a location and an orientation of the sensorunit 160K will be described. The other sensor units 160Y, 160M and 160Chave the same configuration as that of the sensor unit 160K.

FIG. 5 is a diagram which illustrates an example of a location and anorientation of the first sensor 161K included in the sensor unit 160Kwhich is used for horizontal synchronization according to theembodiment. As illustrated in FIG. 5 , the first sensor 161K which isused for horizontal synchronization is disposed so that the longitudinaldirection of the first sensor 161K is orthogonal to the scanningdirection on a substrate 1600K.

FIG. 6 is a diagram which illustrates an example of a location and anorientation of the first sensor 161K and the second sensor 162K, whichis configured to detect a change in scanning position according to theembodiment. As illustrated in FIG. 6 , the second sensor 162K isdisposed on the substrate 1600K at a predetermined angle which isdifferent from the angle of the first sensor 161K, with respect to thescanning direction on the substrate 1600K. The predetermined angle isset to an angle θ with respect to the first sensor 161.

FIG. 7 is a diagram which describes a method of detecting a change inscanning position according to the embodiment.

In FIG. 7 , the reference numeral g1 denotes a scanning positionobtained from the most recent color position shift operation. In thecase of the scanning position of the reference numeral g1, the positionat which the light passes, as detected by the first sensor 161K is a1,and the position at which the light passes, as detected by the secondsensor 162K is b1. A time difference between the detection of light atthe position a1 and the detection of light at the position b1 is t1.

In addition, in FIG. 7 , a reference numeral g2 denotes a scanningposition that is shifted after a time has elapsed since the most recentcompletion of color position shift operation. In the case of thescanning position of the reference numeral g2, the position at which thelight passes, as detected by the first sensor 161K is a2, and a positionat which the light passes, as detected by the second sensor 162K is b2.A time difference between the detection of light at the position a2 andthe detection of light at the position b2 is t2, which is larger thant1.

In addition, the distance between the detected positions of the light inthe case of the reference numeral g1 and the detected positions of thelight in the case of the reference numeral g2 (representing a shift inscanning position in the sub-scanning direction) is set to Δy [mm]. Inaddition, an angle between the first sensor 161K and the second sensor162K is set to θ.

If a scanning speed (i.e., moving speed of the scanned light on thesurface of the photoconductive drum) is set as

$\begin{matrix}{{\Delta\; y} = \frac{v \cdot \left( {{t\; 2} - {t\; 1}} \right)}{\tan\;\theta}} & (1)\end{matrix}$v [mm/s], the change in scanning position in the sub-scanning directionΔy [mm] is expressed by equation (1).

By determining an allowable amount of the shift Δy [mm], that is, anallowable shift amount, it is possible to determine an allowable changein the time difference between the detection of light at a position onone sensor and the detection of light at a position on the other sensor,that is, it is possible to determine a threshold value for the timedifference. In addition, in FIG. 2 , a horizontal direction with respectto a paper denotes a direction of the change Δy [mm] (Δyy, Δym, Δyc, andΔyb). In addition, the allowable amount of the change Δy [mm] is lessthan 2 lines, for example. When a resolution is 600 dpi, one line isapproximately 42 [μm].

The configurations of the first sensor 161K and the second sensor 162Killustrated in FIGS. 6 and 7 are examples, and they are not limited tothese.

FIG. 8 is a diagram which illustrates another example of an orientationof the first sensor 161K and the second sensor 162K according to theembodiment. In FIG. 8 , the direction of the arrow g100 denotes ascanning direction of the scanning light.

A diagram in a region denoted by a reference numeral g101 is an examplein which the second sensor 162K is slightly rotated to thecounterclockwise direction with respect to the first sensor 161K.

A diagram in a region denoted by a reference numeral g102 is an examplein which the first sensor 161K is slightly rotated to the clockwisedirection with respect to the second sensor 162K.

A diagram in a region denoted by a reference numeral g103 is an examplein which the first sensor 161K is slightly rotated to thecounterclockwise direction with respect to the second sensor 162K.

In the information processing device 100, the first sensor and thesecond sensor are provided in respective optical systems correspondingto each of the colors. Alternatively, in the information processingdevice 100, only the first sensor 161K and the second sensor 162K may beprovided in the optical system fora color of which a use frequency ishigh, for example, black. Alternatively, in the information processingdevice 100, the first sensor and the second sensor may be provided in anoptical system corresponding to at least one color that is not black.Alternatively, in the information processing device 100, one sensor unitmay be disposed with respect to any one of optical systems of colors onboth ends which are disposed in the sheet conveying direction. Thecolors on both ends in the sheet conveying direction are, for example, ayellow color or a black color in FIG. 2 , in a case of four colors.

In the examples illustrated in FIGS. 5 to 7 , the pair of sensors aredisposed on one substrate, however, a sensor in which a light receivingunit corresponding to the pair of sensors is provided in one chip may beadopted. In addition, the pair of sensors may be provided on separatesubstrates.

In the examples illustrated in FIGS. 4 to 8 , an example in which thesensor unit is provided as the pair of sensors is described, however,the number of the sensors for one sensor unit may be two or more. Inthis case, predetermined angles of at least two sensors with respect tothe light scanning direction is different, that is, the angle formed bythe two sensors is angle θ.

In addition, an angle θ formed by the first sensor 161 and the secondsensor 162, and the time difference t1 in an initial position of ascanning line may be stored in the storage unit 180 at a time ofshipment, for example. In a case where the first sensor 161 and thesecond sensor 162 are provided in each of the light sources 132, theangle θ formed by the first sensor 161 and the second sensor 162, andthe time difference t1 in an initial position of a scanning line may bestored for each of the light sources 132 in the storage unit 180 at atime of shipment, for example.

Subsequently, examples of signals of the first sensor 161 and the secondsensor 162 at a scanning position after a time of the passage of lightfrom recent execution of color position shift operation will bedescribed.

FIG. 9 is a timing diagram of signals of the first sensor 161 and thesecond sensor 162 at the scanning position after a passage of time fromthe most recent execution of the color position shift operationaccording to the embodiment. In FIG. 9 , a horizontal axis denotes atime, and a vertical axis denotes a level of the signals.

As illustrated in FIG. 9 , a time difference obtained right after themost recent prior execution of color position shift operation, betweenthe detection of light by the first sensor 161 and the detection oflight by the second sensor 162 is t1.

Subsequently, examples of signals of the first sensor 161 and the secondsensor 162 at the scanning position after some passage of time will bedescribed.

FIG. 10 is a timing diagram of signals of the first sensor 161 and thesecond sensor 162 at the scanning position after some passage of timefrom a time the color position shift operation was most recentlyexecuted. In FIG. 10 , a horizontal axis denotes a time, and a verticalaxis denotes a level of the signals.

As illustrated in FIG. 10 , a time difference between the timing of thedetection of light by the first sensor 161 and the timing of thedetection of light by the second sensor 162 is t2.

Subsequently, an example of procedure of processing which is performedby the control unit 170 will be described.

FIG. 11 is a flowchart which illustrates an example of power-onprocessing, which is carried out by the control unit 170 and checks forthe need for color position shift operation according to the embodiment.In the example illustrated in FIG. 11 , it is assumed that colorposition shift operation is performed when the information processingdevice 100 enters a power ON state from a power OFF state, and themeasured time difference t1 between the detection of the light by thefirst sensor 161 and the detection of the light by the second sensor 162is stored in the storage unit 180. In addition, it is assumed that thefirst sensor 161 is disposed so as to be perpendicular to the scanningdirection, and the angle formed by the first sensor 161 and the secondsensor 162 is θ.

(ACT S1) The change amount detecting unit 171 starts counting at afalling timing at which a detection result is changed from an H (high)level to an L (low) level, when the first sensor 161 outputs thedetection result.

(ACT S2) The change amount detecting unit 171 obtains the timedifference t2 in the timing of detecting of light by the first sensor161 and the second sensor 162 by ending counting at a falling timing atwhich the detection result is changed from the H level to the L level,when the second sensor 162 outputs the detection result.

(ACT S3) The change amount detecting unit 171 reads the time differencet1, the scanning speed v, the angle θ formed by the first sensor 161 andthe second sensor 162. Subsequently, the change amount detecting unit171 obtains the change Δy in the scanning position in the sub-scanningdirection by substituting the time difference t1, the scanning speed v,the angle θ which is formed, and the obtained time difference t2 whichare read in the equation (1).

(ACT S4) The control unit 170 compares the change Δy in the scanningposition in the sub-scanning direction which is calculated by the changeamount detecting unit 171 with the first threshold value stored in thestorage unit 180. Subsequently, the control unit 170 proceeds toprocessing in ACT S5 when the change Δy in the scanning position in thesub-scanning direction is equal to or more than the first thresholdvalue (Yes in ACT S4). Alternatively, the control unit 170 ends theprocessing when the change Δy in the scanning position in thesub-scanning direction is less than the first threshold value (No in ACTS4).

(ACT S5) The control unit 170 performs the color position shiftoperation. The control unit 170 then ends the processing after the colorposition shift operation.

In addition, the control unit 170 performs processing from ACT S1 to ACTS5 after an elapse of a predetermined time (for example, one minute) andevery printing ending time, after ending of printing of a predeterminednumber of sheets, at a predetermined point of time and at a time ofending printing, every time of printing one sheet, and the like.

Here, an example of color position shift operation will be described.

As illustrated in FIG. 4 , a case in which the printing unit 130includes the first light source to the fourth light source for fourcolors will be described as an example. When light sources of fourcolors are provided, the information processing device 100 is alsoprovided with four optical systems such as mirrors. In addition, asillustrated in FIG. 2 , mirrors are provided for each color. Inprinting, when printing of only a black color is frequently performed, afrequency of light emitting of the first light source for black colorbecomes high. Due to this, when a temperature of the optical system forblack color becomes higher than the optical system of another color, adifference between the optical systems occurs in opticalcharacteristics, and there is a case in which a shift occurs between thechange Δy1 in the scanning position in the sub-scanning direction of theblack color and the change Δy2 in the scanning position in thesub-scanning direction of another color.

Here, an example of the color position shift operation will bedescribed.

A shift in printing position in each color is caused by any of thefollowing:

I. A wavelength of light is shifted due to a temperature change.

II. A positioning component in the device is subjected to thermalexpansion.

III. A distance between the optical element and the photoconductive drumchanges, for example, as a result of an exchange of any part of theprinting unit 130, or the like, of the photoconductive drum.

In such cases, the control unit 170 instructs the image forming unit 173to form an image so that a distance at respective predeterminedpositions on the upstream side and the downstream side in the transportdirection becomes the same as each other, for example. This technique isdescribed in Japanese Patent Application No. 2017-084732.

In the related art, a control for the color position shift operation, orthe like, is performed at a point of time when the total cumulative timefor continuous or intermittent printing exceeds a preset maximum.

In the color position shift operation according to this embodiment, thecontrol of color position shift operation is described with reference toFIG. 12 . FIG. 13 is a flowchart which illustrates steps of the colorposition shift operation.

The control unit 170 causes the printing unit 130 to form apredetermined first pattern with a first toner (e.g., black toner) and apredetermined second pattern with a second toner different from thefirst toner (e.g., yellow toner), on the surface of the intermediatetransfer belt ITB (ACT 901). The sensors 210 detect the patterns formedon the surface of the intermediate transfer belt ITB which passesthrough the sensors 210 (ACT 902). The control unit 170 calculatesdistances tr1 and tr2 on the basis of output of the sensor 210 at therear side. Further, the control unit 170 calculates distances tf1 andtf2 on the basis of output of the sensor 210 at the front side (ACT903). The printing unit 130 carries out the color position shiftoperation such that tr1, tr2, tf1 and tf2 become equal values byshifting an output timing of emission of the laser light from the lightsources in the sub-scanning direction (ACT 904).

When the color position shift operation is executed at a point of timeat which a preset time elapses, if the preset time is too long, there isa case in which a temperature change in the laser scanning unit becomestoo large due to heat generation of the optical system such as a motorfor rotating the polygon mirror, and a color position shift also becomestoo large, especially when continuous printing is continued. Inaddition, in the related art, when the preset time is too short, thereis a case in which a color position shift operation is too frequentlyexecuted, and productivity of a printing output decreases, or a lifespanof the developer or the photoconductive drum decreases.

According to the above described embodiment, two or more sensors ofwhich angles are different with respect to the optical system of atleast one color, and the change amount detecting unit 171 which detectsa time difference in detecting timing of the sensors are provided. Inthis manner, according to the embodiment, since the color position shiftoperation is performed when a time difference in the timing of lightdetection by the two sensors has changed by a predetermined amount afterthe execution of the most recent color position shift operation, it ispossible to correct for the color position shift at a more proper timingas compared to the related art. In addition, according to theembodiment, it is possible to suppress a decrease in productivity of aprinting output, or a decrease in lifespan of the developer or thephotoconductive drum.

In addition, in the example illustrated in FIG. 11 , an example in whichcolor position shift operation is performed when the change Δy in thescanning position in the sub-scanning direction is equal to or more thanthe threshold value is described; however, it is not limited to this.For example, when the change Δy in the scanning position in thesub-scanning direction is one line or more, and less than two lines, thecontrol unit 170 performs a correction of color position shift ratherthan performing the color position shift operation.

FIG. 14 is a flowchart which illustrates steps carried out to determinewhether to perform color position shift operation or the correctionprocessing. In addition, description of the same processing given inFIG. 11 will be omitted by using the same reference numerals.

(ACT S1 to ACT S3) The change amount detecting unit 171 performs ACT S1to ACT S3. The change amount detecting unit 171 proceeds to processingin ACT S11 after the processing.

(ACT S11) The control unit 170 compares the change Δy in the scanningposition in the sub-scanning direction which is calculated by the changeamount detecting unit 171 with the first threshold value stored in thestorage unit 180. The first threshold value is a value of two lines, forexample. Subsequently, when the change Δy in the scanning position inthe sub-scanning direction is equal to or more than the first thresholdvalue (Yes in ACT S11), the control unit 170 proceeds to processing inACT S5. Alternatively, the control unit 170 proceeds to processing inACT S12, when the change Δy in the scanning position in the sub-scanningdirection is less than the first threshold value (No in ACT S11).

(ACT S12) The control unit 170 compares the change Δy in the scanningposition in the sub-scanning direction which is calculated by the changeamount detecting unit 171 with the second threshold value stored in thestorage unit 180. The second threshold value is, for example, a distancebetween adjacent two scanning lines. Subsequently, the control unit 170proceeds to the correction processing in ACT S13 when the change Δy inthe scanning position in the sub-scanning direction is the secondthreshold value or more, and is less than the first threshold value (Yesin ACT S12). Alternatively, the control unit 170 ends the processingwhen the change Δy in the scanning position in the sub-scanningdirection is less than the second threshold value (No in ACT S12).

(ACT S13) The control unit 170 performs correction processing. Thecontrol unit 170 ends the processing after the correction processing.

Here, an example of the correction processing will be described.

For example, it is assumed that the change Δy in the scanning positionin the sub-scanning direction of a black color is the second thresholdvalue or more, and less than the first threshold value.

The control unit 170 controls the driving unit 131 so as to advance ordelay a driving timing of the first light source (FIG. 2 ) for blackcolor, depending on whether a value of a shift of one line is a positivevalue or a negative value. The control unit 170 may advance or delay adriving timing, by performing a control of shifting a timing of a linebuffer provided in the driving unit 131. The control unit 170 may shifta frequency of a clock signal of phase locked loop (PLL) which is usedwhen controlling the printing unit 130 between printing and printing,and may perform a correction of one line of a horizontal synchronoustiming at the printing start position, similarly.

The control unit 170 performs the correction processing by advancing ordelaying a light emitting timing of the light source 132 in this manner.

In addition, the control unit 170 similarly controls the driving unit131 so as to advance or delay the driving timing of the light source 132depending on whether the value of the shift of one scanning line is apositive value or a negative value, when the change Δy in the scanningposition in the sub-scanning direction of the light source 132 ofanother color is also the second threshold value or more, and less thanthe first threshold value. In addition, when the value of the shift is apositive value, it is a case of being shifted to the right directionwith respect to the paper in FIG. 2 , for example. When the value of theshift is a negative value, it is a case of being shifted to the leftdirection with respect to the paper in FIG. 2 , for example.

According to the above described embodiment, two or more sensors ofwhich angles are different with respect to the optical system of atleast one color, and the change amount detecting unit 171 which obtainsa time difference in the timing of detecting of light by the sensors,are provided. In this manner, according to the embodiment, since thecolor position shift operation is performed when a time differencebetween the light detection by the two sensors is changed by apredetermined amount since the most recent execution of the colorposition shift operation, it is possible to only correct the colorposition shift when the time difference changed by less than thepredetermined amount.

In the above described each embodiment, an example in which the controlunit 170 is a hardware functional portion is described; however, thecontrol unit may be a functional portion which is executed, usingsoftware.

According to the above described at least one embodiment, it is possibleto correct a color position shift by using two or more sensors of whichorientations are different with respect to the optical system of atleast one color, and the change amount detecting unit 171 which measuresa time difference in the timing of detecting light by the sensors.

While certain embodiments have been described these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms: furthermore variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the invention.

What is claimed is:
 1. An image forming apparatus comprising: a polygonmirror configured to rotate to scan a light in a scanning direction; afirst light sensor configured to detect the light reflected by thepolygon mirror, the first light sensor having a longitudinal sidecrossing the scanning direction; a second light sensor configured todetect the light reflected by the polygon mirror, the second lightsensor having a longitudinal side crossing the scanning direction, thelight reflected by the polygon mirror passing both of the first lightsensor and the second light sensor, a light sensing surface of the firstlight sensor being not parallel to a light sensing surface of the secondlight sensor; and a control unit configured to determine a differencebetween a first timing of detecting the light by the first light sensorand a second timing of detecting the light by the second light sensorand perform a color position shift operation based on the difference. 2.The apparatus according to claim 1, further comprising: a third lightsensor configured to detect the light reflected by the polygon mirror,the third light sensor having a longitudinal side crossing the scanningdirection; and a fourth light sensor configured to detect the lightreflected by the polygon mirror, the fourth light sensor having alongitudinal side crossing the scanning direction, the light reflectedby the polygon mirror passing both of the third light sensor and thefourth light sensor, a light sensing surface of the third light sensorbeing not parallel to a light sensing surface of the fourth lightsensor, wherein the control unit is further configured to determine adifference between a third timing of detecting the light by the thirdlight sensor and a fourth timing of directing the light by the fourthlight sensor and perform the color position shift operation based on thedifference between the first timing and the second timing and thedifference between the third timing and the fourth timing.
 3. Theapparatus according to claim 2, wherein the control unit is configuredto perform horizontal synchronization using the first light sensor andthe third light sensor for horizontal synchronization.
 4. The apparatusaccording to claim 1, further comprising: a plurality of laser scanningsystems of different colors arranged in a sheet transport direction, andthe first and second light sensors are positioned in the last one of thelaser scanning systems in the sheet transport direction.
 5. Theapparatus according to claim 1, further comprising: a plurality of laserscanning systems of different colors arranged in a sheet transportdirection, and the first and second light sensors are positioned in thefirst one of the laser scanning systems in the sheet transportdirection.
 6. The apparatus according to claim 1, wherein the controlunit is configured to perform an operation of correcting a drivingtiming of a laser light source, when the difference is less than a firstthreshold value and greater than a second threshold value.
 7. Theapparatus according to claim 1, wherein the light sensing surface of thefirst light sensor is perpendicular to the scanning direction and thelight sensing surface of the second light sensor is not perpendicular tothe scanning direction.
 8. The apparatus according to claim 7, whereinthe second light sensor detects the light that has passed the firstlight sensor.
 9. The apparatus according to claim 7, wherein the firstlight sensor detects the light that has passed the second light sensor.10. The apparatus according to claim 1, wherein the control unitperforms the color position shift operation when the difference isgreater than a predetermined threshold value.